Acquisition Activity - Schedule of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	May 16, 2016	Dec. 31, 2016	Sep. 30, 2016
Liabilities assumed:
Goodwill		$ 739,023	$ 739,023
HF Financial Corporation
Identifiable assets acquired:
Cash and cash equivalents	$ 18,818
Investment securities	165,052
Loans	863,741
Premises and equipment	19,220
Accrued interest receivable	4,117
Other repossessed property	4
Intangible assets	7,877
Loan servicing rights	6,573
Other assets	36,076
Total identifiable assets acquired	1,121,478
Liabilities assumed:
Deposits	863,121
FHLB advances and other borrowings	115,881
Subordinated debentures	21,110
Other liabilities	20,617
Total liabilities assumed	1,020,729
Fair value of net identifiable assets acquired	100,749
Net purchase price	141,965
Goodwill	$ 41,216